Objectives Chemical topping via unmanned aerial vehicles (UAVs) is a crucial technique for large-scale cotton production in Xinjiang. This study aimed to investigate the optimal concentration of topping agents to achieve reasonable control of cotton growth and enhance cotton production efficiency.

Methods A field experiment was conducted in Shihezi City, Xinjiang, using a randomized block design. Five UAV-based chemical topping treatments were established with application rates of: 0.300, 0.525, 0.750, 0.975, and 1.200 L/hm², denoted as H1, H2, H3, H4, and H5, respectively, and manual topping (CK1) and machine topping (CK2) at 0.750 L/hm² concentration were used as controls. During the 0−20 days after topping, plant height, dry matter accumulation and distribution, leaf area boll load (LAB), root-to-shoot ratio (RSR), leaf mass area (LMA), and leaf area index (LAI) were investigated. At harvest, yield components, lint yield, harvest index, and fiber quality were assessed.

Results Twenty days after topping, the concentration of the topping agent sprayed by UAV had no significant impact on cotton leaf age and the number of fruit branches. However, H1 exhibited the significantly higher plant height than H5 and CK1, indicating the poorest effect in controlling vegetative growth. The total dry matter accumulation (TDM), boll dry matter accumulation (BDM), LAB, and LMA of cotton all exhibited a trend of initially increasing and then decreasing with an increase in spraying concentration. The TDM and reproductive organ dry matter ratio (RRDM) were highest under the H3 treatment. There were no significant differences in LMA, RSR, and LAI among the treatments, but LAB and single boll weight under the H3 treatment was the highest. Fiber uniformity, micronaire, specific strength, and fiber maturity of cotton fibers all increased initially and then decreased with an increase in the spraying concentration, while fiber elongation decreased initially and then increased. Average fiber uniformity and specific strength of cotton fibers under the H3 treatment were the highest.

Conclusions The spraying concentration of topping agent is a key factor influencing the effectiveness of UAV-based chemical topping. At appropriate spraying concentrations, UAV spraying better control vegetative growth, promote dry matter accumulation and distribution in cotton bolls, increase single boll weight, and enhance the micronaire, specific strength, and fiber elongation of cotton fibers compared with manual and vehicle-mounted mechanical spraying of topping agents. In summary, it is recommended to use UAVs for spraying chemical topping agents, with recommended application rates of 0.750 L/hm² and 0.975 L/hm².